Мой брат уже думает о том, как будет защищать диплом весной, а после диплома хочет немного отдохнуть и поехать к морю.

Мой брат is the subject of the sentence, so it is in the nominative case.

• мой = my
• брат = brother

Both are masculine singular nominative because they match the role of the subject.

Уже думает literally means is already thinking or is already starting to think.

Russian often uses the present tense where English might also say has started thinking about or is already thinking about. The idea is that this is happening now.

• уже = already
• думает = thinks / is thinking

So Мой брат уже думает... means that even now, before the event happens, he is thinking ahead about it.

This is a very common Russian structure:

думать о том, как... = to think about how...

Literally, о том means about that, but in English we usually would not translate it so directly. It acts as a linking element before the clause introduced by как.

So:

• думает о том = thinks about that
• как будет защищать диплом = how he will defend his diploma/thesis

Together: думает о том, как будет защищать диплом = is thinking about how he will defend his diploma/thesis

Russian often uses this kind of pointer word (том) before a clause.

Because как будет защищать диплом весной is a subordinate clause.

Russian uses commas very consistently to separate main clauses from subordinate clauses. Here the main part is:

Мой брат уже думает о том

and the subordinate clause is:

как будет защищать диплом весной

So the comma is required.

Yes, literally it does, but the meaning is more specific in Russian academic language.

Защищать диплом means to defend one’s diploma project / thesis / final qualifying work. It refers to the formal presentation or defense at the end of study.

Important point: диплом in Russian can mean:

1. the actual diploma document, and
2. the final thesis/project associated with graduation

In this sentence, it clearly means the second idea.

Both are possible in Russian in different contexts, but they are not exactly the same.

• будет защищать = will be defending / will defend
  This is the future of the imperfective, focusing on the process or event as something planned or considered in its unfolding.

• защитит = will defend / will successfully complete the defense
  This is perfective future, which sounds more focused on the result or completion.

Here, because he is thinking about how he will do it, Russian naturally uses the imperfective: как будет защищать диплом
This emphasizes the process, preparation, and manner.

Russian often uses the instrumental case for seasons when meaning in spring, in summer, etc.

So:

• весна = spring
• весной = in spring

This is a standard time expression:

• зимой = in winter
• весной = in spring
• летом = in summer
• осенью = in autumn

So весной is just the normal form for in the spring.

А often connects two parts that are related but somewhat contrasted or set side by side.

Here the sentence has two parts:

1. He is already thinking about defending his diploma in spring
2. After the diploma, he wants to rest a little and go to the sea

Using а gives the sense of:

• and meanwhile
• whereas
• and then
• as for the next part

It is not a strong contradiction, but it marks a shift from one idea to another more clearly than и would.

Because the preposition после always requires the genitive case.

So:

• после = after
• диплом → диплома in the genitive singular

После диплома means after the diploma / after the thesis defense / after graduation-related final work, depending on context.

Russian often leaves out the subject pronoun when it is already clear from context.

The subject is still мой брат, and the verb хочет is singular, so we understand that it is still he who wants to rest and go away.

Russian avoids repeating the subject if there is no need.

Because they depend on хочет.

The pattern is:

хотеть + infinitive = to want to do something

So:

• хочет отдохнуть = wants to rest
• хочет поехать = wants to go

Since both actions are what he wants to do, both appear as infinitives: хочет немного отдохнуть и поехать к морю

Because here the sentence refers to single, complete actions he wants to do in the future.

• отдохнуть = to have a rest, to rest for a while and complete that action
• поехать = to set off / go by transport, as a completed action

Perfective infinitives are very common after хотеть when someone wants to achieve a concrete result:

• хочу прочитать книгу = I want to read the book through
• хочу поехать домой = I want to go home

If you used imperfective forms, the meaning would shift more toward process or habituality.

Немного means a little or for a bit.

So:

немного отдохнуть = to rest a little / to relax for a bit

It modifies the infinitive отдохнуть and shows that he does not want a long rest, just some rest.

Both expressions exist, but they are slightly different in nuance.

• на море usually means to the seaside / to the sea for a vacation
• к морю literally means toward the sea / to the sea

In many contexts, поехать к морю also naturally means go to the seaside. It can sound a little more like going to the seacoast area, literally heading toward the sea.

So in this sentence, поехать к морю is perfectly normal and means something like go to the seaside.

Yes. Поехать comes from the verb family related to going by vehicle or transport.

Very roughly:

• идти / пойти = go on foot
• ехать / поехать = go by transport / set off traveling

In modern usage, поехать к морю is the natural choice for a trip like this, even if the exact transport is not stated.

Because как is the word needed here, not чтобы.

He is thinking about how he will defend his diploma, not about in order to defend it.

Compare:

• думает о том, как будет защищать диплом = he is thinking about how he will defend it
• думает о том, чтобы защитить диплом = he is thinking about defending/passing it, more like considering that goal itself

So как focuses on the manner or way.

Yes. Structurally, it works like this:

Main clause 1: Мой брат уже думает о том

Subordinate clause: как будет защищать диплом весной

Main clause 2: а после диплома хочет немного отдохнуть и поехать к морю

So it is one sentence with:

• a first main clause,
• an embedded subordinate clause,
• and then a second coordinated main clause introduced by а.

That is why the punctuation looks the way it does.